1. Field of the Invention
The present invention relates to a power conversion controller, and more particularly to a power conversion controller capable of providing constant load current control and power factor correction for power conversion applications.
2. Description of the Related Art
In power conversion applications utilizing AC power sources, power factors are often required to be as high as possible. Taking LED driver circuit as an example, when a LED module included in a LED driver circuit is powered by an AC power source, the power factor won't be good unless an appropriate power factor correction mechanism is applied to the LED driver circuit. To include power factor correction in the LED driver circuit, prior art used to adopt CV (Constant Voltage) architecture because it is common seen in switching power applications that power factor correction mechanisms are implemented in CV architectures.
FIG. 1 shows the architecture of a prior art CV (Constant Voltage) LED driver circuit. As shown in FIG. 1, CV function is mainly attained by a LED driver controller 100 and an isolation feedback network 110, wherein an error signal VERROR is delivered by the isolation feedback network 110 to the LED driver controller 100, according to the difference between a DC reference voltage and a division of an output voltage VOUT. The isolation feedback network 110 is typically implemented with a photo-coupler circuit and a programmable shunt regulator. To simultaneously attain CV and power factor correction, the peak value of a current sensing signal VCS is designed to follow the product of VD—a division of a line voltage VIN—and the VERROR, wherein VCS represents the current flowing through the primary side of a transformer. In steady state, VERROR will approach a constant, a rated VOUT is provided thereby; and the peak value of the current flowing through the primary side will follow the line voltage VIN to exhibit a good power factor.
However, since the current-voltage characteristic of LED is of exponential function, a slight voltage variation can cause a large variation of current, resulting in unstable illumination. Therefore, CV is not suitable for application requiring constant load current—such as the LED driver circuit, and there is a need to provide a power conversion circuit including power factor correction in CC (Constant Current) architecture.
In view of this problem, the present invention proposes a novel power conversion controller having an adaptive peak current reference, to not only provide a constant average current, but also correct the power factor.